A spotty-eyed hoverfly: a male Eristalinus sepulchralis (Diptera: Syrphidae). Eristalinus aenus is similar but is a coastal species in the UK and no permanent inland populations have been found (Stubbs and Falk, British Hoverflies, 2nd edition, 2002).
The above image shows the grey lines on the thorax that are often present in E. sepulchralis.
See here for what was probably a female E. sepulchralis that I photographed last summer.
Photos taken on the dam at the lower end of the large lake, Whiteknights Park, Reading University grounds, Reading, UK, on 2011-05-10.